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Molecular Genetics. The Regulation of Gene Expression. The Operon Model. The Operon Model. An operon is a stretch of DNA that contains a set of one or more genes involved in a particular metabolic pathway. An operator is a DNA sequence located within the promoter sequences. .
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Molecular Genetics The Regulation of Gene Expression
The Operon Model • An operonis a stretch of DNA that contains a set of one or more genes involved in a particular metabolic pathway. • An operator is a DNA sequence located within the promoter sequences.
Negative Regulation • Negative gene regulation • This is where a protein interacts directly with the genome to turn off gene expression. • In an E. coli cell, a repressor binds to the operator region upstream from the lacenzymes. • This repressor prevents RNA polymerase from creating an mRNA transcript for the lacenzymes.
Positive Gene Regulation • What happens when glucose and lactose are both available for the cell? • A second mechanism ensures that the lac genes are only expressed at a high level when there is no glucose available in the cell. • If there is no glucose, cyclic AMP (cAMP) will accumulate in the cell. • cAMP will bind to an activator. • This activator will then bind to a site close to the Plac promoter.
Positive Gene Regulation • This is an example of positive gene regulation. • The attachment of the activator makes it easier for RNA polymerase to bind to the promoter. • If more RNA polymerase binds to the promoter, the lacgenes will be transcribed at a higher rate. • Inducer molecule – a molecule that promotes transcription. • Because an inducer plays a role in both positive and negative gene regulation in the lacoperon, it is said to be an inducible operon.
Co-repression in the trypOperon • Under normal conditions, an E. coli cell produces the enzymes necessary to synthesize the amino acid tryptophan. • However, if the cell already contains more than enough tryptophan, the cell does not need to waste any energy making more. • If tryptophan levels are high, tryptophan molecules will bind to a repressor protein and activate it. • The activated repressor protein will then bind to the operator and prevent transcription. • This is considered a repressible operon.
Gene Expression in Eukaryotes • Eukaryotic organisms often contain multiple tissue types and so, unlike prokaryotes, they do not need to be expressing large amounts of their genome in every cell. • As a result, eukaryotes are more dependent on mechanisms that keep gene expression turned off most of the time, and that turn on selected genes only as they are needed.
Gene Expression in Eukaryotes • Pre transcriptional control • What DNA is exposed to transcription enzymes? • Transcriptional control • What parts of the exposed DNA is transcribed into pre-mRNA? • Post-transcriptional control • How much pre-mRNA is processed into finished mRNA?
Gene Expression in Eukaryotes • Translational control • How much of the mRNA is actually transported to ribosomes within the cytoplasm? • Post-translational control • Once the polypeptide is complete, how and when will the cell modify it to become a functional protein? • Will the cell allow the polypeptide to be broken down before it becomes a functional protein?
